The object of the present invention is to provide a red colored quinacridone pigment of high saturation which has the heat stability and weatherability for utility as a replacement for heavy metal pigments in engineering plastics. This object is achieved by the preparation of red colored pigment compositions by a process comprising the steps of premilling a pigment mixture of 2,9-dichloroquinacridone pigment and bis-(4,5,6,7-tetrachloroisoindolin-1-on-3-ylidene)-phenylene-1,4-diamine pigment, and subsequently heating a suspension of the premilled pigment mixture in a polar solvent to an elevated temperature.
Quinacridones are valuable pigments. The gamma crystal modification of the unsubstituted quinacridone, C.I. Pigment Violet 19, is known for its attractive red color. Although it is of coloristic interest, the red-colored gamma-phase of unsubstituted quinacridone possesses insufficient heat stability to have utility for pigmenting the engineering plastics.
The U.S. Pat. No. 5,095,056 discloses a pigmentary 2,9-dichloroquinacridone having a specific surface area below 30 m.sup.2 /g with good heat stability. Due to its very large particle size, the color is shifted to a unique red color shade from the known magenta color of smaller particle size 2,9-dichloro-quinacridone. However, its color is still in the magenta region and readily distinguishable from red the color shade of an unsubstituted gamma quinacridone. Thus, no red quinacridone pigment is known to possess the heat stability required for utility as a pigment for the engineering plastics.
Isoindolinone pigments, particularly C.I. Pigment Yellow 110, are known to possess good heat stability and good weatherability.
EP 042,816 describes a process for the preparation of pigment alloys by submitting pigment mixtures containing at least two pigments of the perylene, phthalocyanine, perinone, quinacridone, quinaphtholone, isoindolinone, isoindoline, dioxazine, anthraquinone, thioindigo, azo, and azo methine pigment class to a wet milling process. However, mixtures of 2,9-dichloroquinacridone and a isoindolinone pigment which are prepared according to the disclosed procedures are very small particle size pigments which possess insufficient heat stability for use in engineering plastics.
The present invention is based on the discovery that red pigment compositions, which possess high saturation, excellent weatherability and the outstanding heat stability required for utility as a pigment in engineering plastics, can be obtained according to the present process. In addition, the pigments prepared according to the present process are highly dispersible in the plastic medium to be pigmented. Thus, the inventive compositions provide a red quinacridone pigment which is suitable for the utility of pigmenting engineering plastics.